Conveyor belts are used in order to transport goods, e.g. during their production or processing, or in order to deliver them from one starting point to a point of use. A most common conveyor belt is constructed of one or more traction layers, such as of a woven fabric, to impart the belt the required tensile strength, and one more plastic or rubber layers that are arranged on top of the traction layer(s) an/or in between them. In order to tailor a conveyor belt to certain conditions of use, such as conditions where high chemical resistance, resistance to microbial fouling, and in particular also resistance to abrasion, is required, it has been customary to apply as the outermost layer a layer of a material having the respective property and which thus imparts the overall belt that property. Specifically for the purpose of imparting the belt resistance to abrasion it has been customary to apply onto the belt a cover layer of crosslinked polyurethane or of TPU (thermoplastic polyurethane, thus essentially or even entirely not-crosslinked), these being materials which as such are known to have a high abrasion resistance.
In particular when the conveyed good is paper or cardboard, such as mail, it has been observed by the inventors of the present application that belts with prior art crosslinked polyurethane or TPU surfaces and without rubber covers are prone to “glazing”, i.e. they lose surface roughness by the fine polishing action of the paper mail. A “glazed” belt surface is a shiny, polished surface of lowered static friction coefficient with, thus of impaired transporting capacity for, the paper mail. The polishing action arises because there is some relative motion between the belt's top surface and the mail conveyed by it, thus giving rise to friction and concomitant abrasion. Specifically, in a mail sorting machine there are situations where the mail is transported in a sandwiched form between two belts that cooperate with each other by embedding the mail between the top surfaces of the two belts. If in such a dual belt conveying setup the sandwiched mail must be transported along a curve there are invariably speed differences between the inner belt and the outer belt. That speed difference increases with increasing thickness of the mail items. Said speed differences, thus relative motions, may cause friction between the inner belt's top surface, the transported mail and the top surface of the outer belt, with concomitant increased abrasion and “glazing” of the two belts. Consequently the grip between transported goods and the belt decreases and goods are no longer precisely positioned and transported (and/or sorted).
Also in particular with belts used for power transmission, it has been observed by the inventors of the present application that belts with prior art crosslinked polyurethane or TPU surfaces and without rubber covers are prone to the same “glazing”. In power transmission applications it often occurs that the belt is in a serpentine-like arrangement, with alternatingly either the belt's first top surface or second top surface coming into contact with, and being bent over, a pulley. In such a serpentine-like arrangement both the belts first and second top surfaces are equally required for transmission of power. By virtue of said “glazing” occurring on one or, in the serpentine-like arrangement, even on both top surfaces, belt slippage will occur and the belt has to be re-tensioned in order to increase the shaft load above the critical limit which is required for transmission of power without slippage.
CA 1021509 A discloses a elastomeric, thus crosslinked polyurethane foam conveyor belt. The foam is abrasion-resistant, but has a solid or nearly solid so-called “skin” on its surface (i.e. a surface portion with near-zero foaming degree), and the publication hints at that it is that skin that retains at the foam's surface the abrasion resistance of the unfoamed polyurethane.
DE 37 10 160 A discloses a conveyor belt having a foamed cover layer 12 which may be of foamed polyurethane. There is no disclosure as to whether it is an elastomeric or thermoplastic/thermoplastic elastomeric polyurethane. The foam should again cover itself with a “closed skin”, or is covered by an explicit polyurethane skin 14 (column 6, lines 11-19). The plastic material of the cover layer 12 having such skin is said to have a “flexible, abrasion resistant surface”.
U.S. Pat. No. 4,752,282 discloses a flat drive belt having a symmetrical construction around a central traction layer and a method of transmitting rotary motion from one pulley 27 to another pulley 30 using that belt.
WO 00/44821 discloses low density foamed TPU's which were blown using a few percent of expandable microspheres, optionally in combination with other exothermic or endothermic blowing agents. The foams have a “relatively thin skin”. In the abrasion tests of table 2 it was observed that TPU foams blown with solely expandable microspheres had a lower abrasion resistance (more abraded material) than the TPU foams blown without expandable microspheres, even when blown to very similar foaming (comparative examples 1 and 3, example 4).
WO 2005/026243 discloses mixtures of TPU with EPM rubber or with modified EPM rubber blown with expandable microspheres and optional other blowing agents. This publication hints at the bottom of page 2 that foaming pure TPU to a density of less than 1.0 g/cm3 is at the expense of its abrasion resistance. Tested was the abrasion resistance of, among others, a) pure TPU blown with only expandable microspheres, b) pure TPU blown with expandable microspheres and chemical blowing agent, c) TPU/EPM-rubber mixture blown with only expandable microspheres, and d) TPU/EPM-rubber mixture blown with expandable microspheres and chemical blowing agent (entries R2, R1, V1.3 and V1.1, respectively, in table 1); all these were blown to very similar densities. The observed amount of abraded material increased (thus the abrasion resistance decreased) in the order d), c), b), a).
WO 2008/113195 A discloses a treadmill belt wherein a first foamed layer of a thermoplastic foam is arranged on top of a first traction layer. In the simplest construction the first foamed layer 12 may be the cover layer of the belt. The thermoplastic of the first foamed layer may be TPU. This publication does not say anything about abrasion resistance of the thermoplastics.
WO 2010/103096 A discloses a belt with a foam layer of a thermoplastic elastomer of a Shore A hardness of at the most 85 in the unfoamed state, which may be TPU. That foamed layer preferably forms the cover layer (page 8, lines 18-24). Two belts with a foam cover layer of a “blend of elastomer and TPE-U (Versollan RU 2204X) Shore A 55 unfoamed” and “polyester-based TPE-U (Laripur 70251) Shore A 70 unfoamed”, respectively, were tested for several physical parameters, among which the “nose bar test”. The test comments on the abrasion, but in view of the way a nose bar test is done this could only relate to the abrasion of a backside fabric traction layer present in the tested belt, that fabric being in contact with the nosebar.
U.S. 2006/0163042 discloses in examples 1 and 3 a butt-weldable conveyor belt having only one (central) fabric traction layer and one layer of polyester-based unfoamed TPU (Estane 58277) on each side of the traction layer. The layer construction of this belt is thus symmetrical about the central traction layer.
U.S. Pat. No. 3,880,272 discloses a belt with two foamed cover layers, wherein the material of the foams may be plastic or polyurethane. It also discloses a conveyer system where two belts, each having a foamed layer, are in contact with each other and encase the goods to be conveyed within their foamed layers.
The applicant of the present application marketed at the time of filing of the instant application special types of conveyor belts which are used for conveying mail and for sorting it, called “machine tapes”, exemplary ones having type codes MAB-02, MAB-05 and MAB-8E. The first two consist of a top (mail conveying) layer of unfoamed TPU and of a back (pulley side) layer of an unfoamed crosslinked polyurethane. These two machine tapes are thus of unsymmetrical layer construction. They do not contain any fabric layer. The third machine tape has a symmetrical construction with a fabric layer laminated on both sides with TPU films.
The instant invention seeks to provide a further abrasion-resistant belt that is in particular suitable for use as a machine tape or in power transmission.